A demand for batteries as a power source is increasing in line with development of portable devices such as personal computers and mobile phones.
Batteries for these devices are expected to be used at normal temperatures. In addition, high energy density and good cycle characteristics are demanded.
To meet this demand, a new active material with high-capacity density has been developed for a positive electrode and negative electrode, respectively. In particular, there is a great interest in using elemental silicon (Si) or tin (Sn), its alloy, or its oxide as a negative active material.
However, rechargeable lithium batteries using these negative active materials do not demonstrate a sufficient charge/discharge cycle characteristics. More specifically, silicon oxide SiOx (x=0.3) is used as a negative active material. Lithium cobalt oxide (LiCoO2) is used as a positive active material, which is generally used for rechargeable lithium batteries. As electrolyte, a mixed solution of ethylene carbonate (EC) and ethylmethylcarbonate (EMC) containing lithium hexafluorophosphate (LiPF6) is used. These materials are used for manufacturing a rechargeable lithium battery with winding the electrodes. This battery shows a significant deterioration in its capacity when charging and discharging is repeated for about 100 cycles under next conditions: 1.0 C of charge/discharge current, 4.2 V of charge cutoff voltage, and 2.5 V of discharge cutoff voltage.
To prevent this deterioration, a discharge potential of the negative electrode is preferably kept as low as possible. For example, Patent Document 1 discloses suppression of deterioration in capacity due to charge/discharge cycles by controlling the discharge cutoff potential of the negative electrode to 0.6 V or lower relative to a reference electrode of lithium in a rechargeable lithium battery using silicon oxide (SiO) as the negative active material.
However, if the reference electrode of lithium is provided in the battery in order to control discharge potential of the negative electrode made of SiO relative to the reference electrode, as in Patent Document 1, a battery structure becomes complicated, causing a disadvantage in practical use. In other words, a third electrode terminal, in addition to positive and negative electrode terminals, becomes necessary in the battery. Still more, if the charge/discharge cycle is repeated from an initial period when there is no deterioration in positive and negative active materials, potential ranges in which positive and negative electrodes are used, respectively, will change as positive and negative active materials are deteriorated. Therefore, even if the discharge cutoff potential of the negative electrode is set to 0.6 V or lower relative to the positive electrode, it is difficult to confirm whether the discharge cutoff potential of the negative electrode is actually maintained at 0.6 V or lower. Accordingly, it is difficult to suppress deterioration in capacity due to repeated charge/discharge cycles by detecting the charge/discharge state of the negative electrode based on measurement of the battery voltage.    Patent Document 1: Japanese Patent Unexamined Publication No. H11-233155